Such an actuator is known from U.S. Pat. No. 6,476,702, for example.
In the case of such actuators, the first conducting element and the second conducting element usually consist of a ferromagnetic material of high permeability. Such actuators contain an oscillatory mass-spring system, which is excited so as to produce oscillations when an alternating current is driven through the turns of the electrically conductive coil.
The at least one magnet has a magnetization with a magnetization direction which is ideally perpendicular to the longitudinal axis of the coil. If a current now flows through the coil, a Lorentz force acts in the direction of the longitudinal axis of the coil. As described in U.S. Pat. No. 6,476,702, the interaction of the magnetic lines of force emerging from the collar-like projections of the first conducting element and the second conducting element or the magnetization of the magnet, which advantageously likewise consists of a material of high permeability, results in a further force, which acts in the same direction as the described Lorentz force. Since the magnetic lines of force are conducted in specific directions by the first and second conducting elements, both component parts are in this case referred to as conducting elements.
In the case of an actuator of the generic type, either the coil with the first conducting element or the magnet with the second conducting element is mounted in a sprung manner, while the respective other assembly is mounted statically. If a current now flows through the coil, the abovementioned forces result in a shift in the spring-mounted assembly and therefore in a movement of the actuator. In this way, valves can be opened or closed, for example. If an alternating current flows through the coil instead of the direct current, the direction of the acting forces reverses along with the current flow direction. In this way, the spring-mounted assembly is caused to oscillate. By targeted selection of the amplitude, frequency and phase of the applied alternating current, the oscillation of the actuator can be controlled very precisely. In this way, for example, oscillations can be produced or an oscillation in phase opposition can be superimposed on already existing oscillations and these already existing oscillations can thus be compensated for.
One disadvantage is the fact that the excitation force that can be achieved with the actuator in accordance with the prior art is relatively low in relation to the physical volume required for this. In addition, the usable frequency range in which the actuator can be operated on alternating current is subject to restrictions. Firstly, the first natural frequency of the system, which is the lowest frequency at which the actuator can be operated, cannot be shifted to lower frequencies. Secondly, the working range in the higher-frequency range is limited owing to the low amount of coil installation space and the use of coils with a small wire diameter associated therewith.
It is known from the prior art to form the first and the second conducting elements of the actuator as a laminate stack comprising many thin layers of ferromagnetic laminations with high permeability in order to reduce losses as a result of ring currents which are produced around the magnetic lines of force. For this purpose, for example, electric steel laminations or iron laminations which are separated from one another by thin insulating layers are suitable. In this way, ring currents around the magnetic lines of force can only result in orders of magnitude of the thickness of these laminations. Although the losses are thus reduced and therefore the efficiency of the actuator can be increased, the achievable excitation forces are still very low in relation to the physical volume.
U.S. Pat. Nos. 6,244,526 and 7,053,741 have disclosed the use of an electromagnetic actuator as a fuel injection valve. However, high excitation forces are not required for this, with the result that the force required for switching a valve can be applied without any problems by an already described actuator. In addition, owing to the special configuration of the actuator in said documents, the radial extent of the actuator with respect to the longitudinal axis of the coil is markedly reduced. That is to say that the movable portion of the actuator is arranged below the statically mounted portion in both documents, with the result that the radial extent is naturally markedly reduced. However, the required volume is still relatively large, even when it now has a different shape than in the case of the first-described embodiment of an actuator. Such an arrangement is unsuitable for operation on alternating current, however.
In addition, the working principle of the actuators described in said documents is different. Both actuators manage without any magnets. The excitation force is only produced by the magnetic field produced by the coil and not, as is the case in this case, also by a current which is flowing in the magnetic field of the magnet and the Lorentz force caused hereby.